SDF internals, ray consensus, improved intersections performance

[clbarnes]

Supercedes #34

[clbarnes]

Downstream users should note:

• threads is now a keyword-only argument for the query methods
• validate should now be a Validation enum rather than a bool
  • we try not to use validation from the trimesh library, instead using the validation built into the underlying rust
  • Validation.min() does the minimum required, Validation.default() == Validation.all() which does all. Validations are intflags which should be combined with |.
• for Volume.contains:
  • you can now set the number of rays used (<= the number given on instantiation). If None, uses all, if < 1, doesn't use rays at all and does a proper containment check, which is usually slower and depends on a valid mesh.
  • you can now set the number of rays which must hit a backface in order for a point to be considered internal. Previously it was all, now it's by default (None) a simple majority of the rays cast. There are short circuits internally to minimise ray casts where possible.

[clbarnes]

@schlegelp @ceesem , probably best to pin your ncollpyde usage to <0.20 for now, then that can be bumped when the usage is updated.